More and more Syrian rebels are defecting to join the ultra-hardline Islamic State of Iraq and Syria insurgency, said U.N. human rights investigators on Friday, in what they described as a “Syrian-ization” of the al Qaeda offshoot.

Members of the independent Commission of Inquiry on Syria, set up by the U.N. Human Rights Council in September 2011, informally briefed the United Nations Security Council on Friday ahead of submitting their latest report next week.

Brazilian chief investigator Paulo Pinheiro said the ISIS was undergoing a “Syrian-ization.”

“What began with a lot of foreign fighters, now you have authentic Syrian citizens,” Pinheiro told reporters. “We are seeing more Syrians coming to ISIS than before.”

He said there was an estimated 10,000 to 15,000 foreign fighters with various groups battling Syrian President Bashar al-Assad’s troops.

Karen Koning AbuZayd, a member of the commission, said most of the Syrians joining Islamic State were defecting from other armed opposition groups fighting in the country’s civil war, now in its fourth year.

“They see it’s better, these guys are strong, these guys are winning battles, they were taking territory, they have money, they can train us,” AbuZayd told reporters.

After a successful offensive in Iraq, the Islamic State last month declared an “Islamic caliphate” in territory it controls in Iraq and Syria and vowed to expand.

Pinheiro said the group was a “good candidate” to be added to the inquiry’s confidential list of suspected war criminals in Syria.

He said his message to the 15-member Security Council on Friday was: “Please do not forget Syria.”

U.N. investigators have not been able to enter Syria to carry out their work. They rely on interviews with refugees, telephone and Skype interviews with victims and witnesses in Syria, and photos, videos, satellite images, and forensic and medical reports.

Corroboration

They seek corroboration to a level where they have “reasonable grounds to believe” an incident occurred.

“Once again we heard of the horrific scale of violations and abuses being carried out against innocent civilians,” British U.N. Ambassador Mark Lyall Grant said after the briefing, which Britain arranged. “All perpetrators should be held to account.”

The inquiry has called for the situation in Syria to be referred to the International Criminal Court. But an attempt by Western members of the council to do so was vetoed by Russia, a close ally of Syria, and China in May.

Without naming names, Pinheiro criticized “influential states” for their ambiguous position of supporting accountability for crimes in Syria and a push for a political solution, while at the same time helping the warring parties.

He said the full range of humanitarian breaches and gross rights violations were being committed by parties in Syria, including torture and summary executions.

“We’re not in a position to say who is winning the world cup of human rights violations,” Pinheiro said.